Chain Surveying- Introduction and Basic terms

                  Chain Surveying

It is part of surveying in which whole area of traverse divided into network of triangles. Then the sides are measured with the help of chain and tape. There is no need for angular measurement. The basic purpose for triangulation is that it is simplest figure that can be plotted form only length. Sometime chain surveying is also known as Chain Triangulation.

Common Terms in Chain Surveying

1.Frame Work
The traverse in which whole area is covered by a set of triangles to be surveyed is called Frame Work. The equilateral triangle is preferred than obtuse-angled triangle as equilateral triangle is plotted more precisely. The triangle in which angles are neither less than 30° and more than 120°  are known as Well Conditioned or Well Shaped Triangle and preferred for surveying.

2.Survey Station
The end point of a line of traverse refer as Survey Station. The line joining the survey station are called survey lines. It is of two types:
(a)Main Survey Station: The station which is present on end of main survey line. The point A, B, C, D, E in below traverse are Main Survey Stations.

(b)Tie or Subsidiary Station: The station which is present on Survey Line. The main purpose is to plot more precise detail in or around the traverse. The a, b, c, d are Tie Station.

Selection of Survey Station 

1. From any Main Survey Station, at-least two Main Survey Station should be clearly visible.
2. It should form well conditioned triangles as far as possible. 
3. Atleast one check line should be provided in traverse.
4. The Survey line should be parallel and close to main building according to maximum possibility. It give more precise distance of offset and also decrease work stress.
5. If traverse is of large size, then whole area is divided into sub parts such as triangle by Tie lines. It increase accessibility of all points. 

3.Base Line

These line generally the longest line of traverse and entire framework is built around it. It generally run in the center of traverse. For larger Traverse, two Base Line are taken for more accuracy. In above traverse, line AFD is base line of traverse ABCDE.

4.Check Line

These line check the accuracy of any plotted traverse. It is line which runs between apex of triangle to another fixed points of triangle. The length of check line measured on ground should match with its length on plan.The line cd, bD, Ab are check lines.  

5.Tie Lines

The line which is used for plotting more detail inside the Traverse. It also serve as Check line for any traverse. It help in avoiding long offsets. The check lines in above traverse also used as Tie lines.

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